Beta decay is a form of radioactive decay in which the nucleus of an atom undergoes a change which causes it to emit a beta particle. The other types of radioactive decay are alpha decay and gamma decay. In addition to being a topic of general scientific interest, beta decay has a number of practical applications, especially in the field of medicine, where beta particles are sometimes used to shrink or kill tumors. Beta decay can also be utilized in medical imaging studies as a tracer.
Atoms undergo beta decay when they are unstable because they have too many neutrons or too many protons. To stabilize themselves, the excess neutrons or protons are converted, conserving mass and making the nucleus more stable. In the process, the atom also changes into another element, because while the overall number of particles in the nucleus remains the same, the balance of protons and neutrons changes.
In beta minus decay, an excess neutron becomes a proton, and the nucleus emits an electron and an antineutrino. The electron is the beta particle, while the antineutrino is a particle with some unusual properties. The existence of neutrinos and antineutrinos was posited as early as the 1930s, but it wasn't until the 1950s that researchers were able to prove the existence of such particles. Three flavors of neutrino have been identified, along with corresponding antineutrinos. (And yes, “flavor” is a technical term in the wide world of particle physics.)
When a nucleus undergoes beta plus decay, a proton is converted into a neutron, with the nucleus emitting a positron and a neutrino. Beta particles can be electrons or positrons, as illustrated, depending on whether a nucleus goes through beta minus or beta plus decay. Before researchers realized that beta particles were just electrons or positrons, they referred to these particles as “beta rays,” which is why some antiquated texts contain references to beta rays.
A beta particle has more penetrating power than an alpha particle, but less than a gamma particle. Beta particles can be stopped with a thick sheet of metal, a large pocket of air, or several sheets of paper. This makes them relatively safe to work around, as long as safety precautions are observed when people are around elements which undergo beta decay.
Like alpha particles, however, beta particles can wreak havoc if they get inside the body. Sometimes, this trait is used to advantage, as when radioactive elements are introduced to the body to treat a cancer. In cases where beta particles are not introduced by design, however, they can damage the cells of the body or even cause reproductive health problems by interfering with the egg and sperm cells.